For the purpose of adding red tone to the emission color of white light emitting diodes (LED) to lower the overall color temperature or improve color rendering, there is a need for phosphors capable of red emission upon excitation with light corresponding to near-UV to blue LED. While research works have been made thereon, JP-A 2009-528429 (Patent Document 1) describes that a complex fluoride phosphor material is obtained by doping a useful complex fluoride of the formula: A2MF6 (wherein A is Na, K, Rb or the like and M is Si, Ge, Ti or the like) with Mn.
As an improvement over the method of Patent Document 1, the inventors developed in JP-A 2012-224536 (Patent Document 2) a method for preparing a complex fluoride of the formula: A2MF6 by the steps of dissolving an oxide, hydroxide, carbonate or fluoride of tetravalent element M such as Si or Ti in hydrofluoric acid to form a first solution containing a fluoride or polyfluoro-acid salt of tetravalent element M in a substantial sense, providing a second solution containing an alkali fluoride AF (wherein A is K, Na, Li or the like) such as potassium fluoride or potassium hydrogenfluoride, mixing the first solution with the second solution to form a precipitate of complex fluoride, and recovering the precipitate by filtration or another technique; and a method for preparing a phosphor of the formula: A2MF6:Mn by the step of previously adding a Mn compound of the formula: K2MnF6 to at least either one of the first solution and the second solution.
On the other hand, a possibility is pointed out that these complex fluoride phosphors degrade their luminescent properties under high temperature/humidity conditions. Referring to the possibility, JP-A 2009-280763 (Patent Document 3) describes that the problem of humidity resistance is mitigated if the LED manufacture process is modified by mixing a phosphor with a silicone resin and molding the resin. Also JP-A 2010-045328 (Patent Document 4) describes that the problem of humidity resistance is mitigated by coating a complex fluoride phosphor with a resin or the like prior to the manufacture of LED.
In the LED manufacture process, however, the phosphor is used at varying stages and under varying conditions, and the form of phosphor used in LED products widely varies. In some cases, it is difficult to apply the above countermeasure. It is thus desirable that the humidity resistance of a phosphor powder itself be improved, that is, a lowering of emission intensity in a high humidity atmosphere be minimized.